java.lang.ClassCastException : java. util.HashSet 无法转换为自定义类

Question

FATAL EXCEPTION: AsyncTask #2
Process: my.app, PID: 15441
java.lang.ClassCastException: java.util.HashSet cannot be cast to my.app.ChatNotificationHashSet
at my.app.UserHandler.getChatNotificationMessage(UserHandler.java:127)
at my.app.NotificationUtil.getChatTitle(NotificationUtil.java:100)
at my.app.NotificationUtil.showChat(NotificationUtil.java:71)
at my.app.service.GCMListenerService.onMessageReceived(GCMListenerService.java:47)
at com.google.android.gms.gcm.GcmListenerService.zzq(Unknown Source)
at com.google.android.gms.gcm.GcmListenerService.zzp(Unknown Source)
at com.google.android.gms.gcm.GcmListenerService.zzo(Unknown Source)
at com.google.android.gms.gcm.GcmListenerService.zza(Unknown Source)
at com.google.android.gms.gcm.GcmListenerService$1.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)

ChatNotificationHashSet

public class ChatNotificationHashSet<E> extends LinkedHashSet<String> {

    @Override
    public String toString() {
        // ...
    }
}

UserHandler(发生异常的地方)

public ChatNotificationHashSet<String> getChatNotificationMessage() {
        return (ChatNotificationHashSet<String>) pref.getStringSet(CHAT_NOTIFICATION_MESSAGE, null); // <- Exception occurs here
    }

    public void setChatNotificationMessage(ChatNotificationHashSet<String> messages) {
        SharedPreferences.Editor editor = pref.edit();
        editor.putStringSet(CHAT_NOTIFICATION_MESSAGE, messages);
        editor.commit();
    }

---

怎么会发生这种事？类型转换时应该不会有任何问题null至ChatNotificationHashSet<String> ， 正确的？我想不出还有什么问题。

Answer 1

getStringSet() 返回的 Set 不需要与传递给 SharedPreferences.Editor.putStringSet() 的相同> 方法。这通常取决于内部实现。我假设它会序列化数据并将其放入某些存储中，并在检索时反序列化。

如果您需要特定的结构，最好实现一些包装器，它可以读取构造函数中的所有设置，或者根据需要从底层 Set 检索设置。